1. Field of the Invention
The present invention relates to a digital moving picture encoding system. More particularly, this invention is concerned with a moving picture hybrid encoding method of dividing an input picture into a plurality of blocks and carrying out motion compensation and orthogonal transformation block by block.
2. Description of the Related Art
A conventional moving picture encoding technique includes a technique stipulated in the international standard H.261 recommended by the Telecommunication Standardization Section of the International Telecommunication Union (ITU-T) and techniques stipulated in the international standards MPEG1 and MPEG2 recommended by the Moving Picture Experts Group (MPEG) (described in the ISO/IEC 11172/13818). These techniques. are generically referred to as a moving picture hybrid encoding method. The fundamentals of the method will be described briefly.
FIG. 1 shows an example of the basic configuration of a conventional moving picture encoding system.
In FIG. 1, a predictive error signal generation means 10 divides an input picture into L rows and M columns of blocks, and calculates by each block of the input motion a prediction error (difference) between the input picture and a predictive picture (0 in an intra case) produced by a prediction parameter (motion vector) calculation means 19 and predictive picture production means 18, which will be described later.
Discrete cosine transform (DCT) that is one kind of orthogonal transformation is performed block by block on the prediction error signal by an orthogonal transformation means 11. Coefficients resulting from DCT and indicating information of the picture (energy distribution) are compressed and quantized appropriately by a quantization means 12. An entropy encoding means 13 carries out reversible encoding using the Huffman code or the like so as to encode quantization coefficients resulting from DCT, a quantizer, prediction parameters (motion vectors), and the like according to the type of a transmission line, and then sends the resulting codes to the transmission line.
A signal quantized by the quantization means 12 is subjected to inverse quantization by an inverse quantization means 14 and to inverse DCT (IDCT) by an inverse orthogonal transformation means 15, whereby the prediction error signal is restored. A decoded picture production means 16 adds the restored prediction error signal and the predictive picture produced by the predictive picture production means 18 so as to restore an original input picture, and stores the original input picture temporarily in a decoded picture memory means 17 so that the original input picture reconstructed can be used as a predictive picture for subsequent input pictures. A prediction parameter calculation means 19 calculates and extracts prediction parameters for a subsequent picture by comparing an original input picture with the decoded input picture. The predictive picture production means 18 produces a subsequent predictive picture on the basis of the results of calculation and the predictive picture stored in the decoded picture memory means 17.
According to the foregoing moving picture hybrid encoding method, a large amount of information is produced when encoding a scene change in which the contents of a screen are changed instantaneously or rapid motion occurs in a moving picture. An amount of information assigned to each frame may exceed a reference value. In this case, the quality of a picture to be transmitted may deteriorate greatly or too many frames may be abandoned. This results in a poor subjective impression.
To avoid the above phenomenon, it has been a matter of common practice in the past that several pictures having different resolutions are produced by converting an input picture and switched properly. This method of switching several pictures has been disclosed in, for example, Japanese Unexamined Patent Publication No. 7-30901 or 7-95566. Prediction parameters (motion vectors) calculated by the prediction parameter (motion vector) calculation means 19 are used to calculate an average magnitude of all the motion vectors. Based on the magnitude, one of the plurality of resolutions is selected and an associated picture is sent over a transmission line.
However, the prior arts disclosed in the Japanese Unexamined Patent Publication Nos. 7-30901 and 7-95566 have the problems described below.
Specifically, there is a problem that when an average motion vector is large but an amount of information to be transmitted is small, the selection of an unnecessarily low resolution invites deterioration of picture quality.
For example, when a whole picture in a screen simply moves in the one direction, an average vector is large and motion prediction itself is carried out efficiently. Moreover, a variation among vectors is small so that an amount of produced information is small. In this case, according to the foregoing prior art, there arises a problem that although it is possible to transmit a high-quality picture having a high resolution, the picture is switched to a picture of deteriorated quality.